The invention relates to a dual band antenna for a handset. Such an antenna includes a metallic plate or layer acting as ground plane for the antenna, a resonator plate or layer acting as radiating element(s), and a feeding point supplying the signal to the antenna. When the ground plane and the resonator plane are electrically short-circuited the feeding point will be placed in a position where the antenna is matched to the RF output of the handset. Such antennas are known as Planar Inverted F-Antennas (PIFA).
Until a few year ago all phones for cellular communication were equipped with an extendable antenna element, as known from e.g. the phone sold under the tradename Nokia 2110.TM.. Later on this extendable antenna element was substituted by an external helix antenna, as known from e.g. the phone sold under the tradename Nokia 6110.TM.. Recently the applicant has launched a phone sold under the tradename Nokia 8810.TM. and this phone includes an internal antenna based on the PIFA concept. The antenna is a so-called single band antenna and the present version it is adapted for GSM in the 900 MHz band (uplink 890-915 MHz and downlink 935-960 MHz). The antenna element will have an electrical length corresponding to a quarter wavelength and by placing a dielectric material between the ground and the resonator plane the overall physical dimensions are reduced. The overall dimensions of the PIFA are reduced to 32.times.20.times.4 mm.
WO 95/24746 describes an internal antenna having a dielectric body coated with a metallic layer on two substantially parallel surfaces. This antenna is a single band antenna for the GSM 900 MHz band only. Basically a plastic body is moulded and coated with metal. Afterwards a pattern is created in the metallic layer by removing parts of the coated surfaces by milling. This concept has been used in the phone marketed by Hagenuk under the tradename Global Handy.TM..
U.S. Pat. No. 5,764,190 describes a capacity loaded PIFA according to which an extra plate is interposed in between the ground plane and the radiating element. This requires that a two-shot moulding process be used in addition to several coating processes.
A letter by C. R. Rowell and R. D. Murch, "A Compact PIFA suitable for dual frequency 900/1800 MHz operation", is published in IEEE Transactions on Antennas and Propagation, April 1998, Volume 46, Number 4. This letter is written by the inventors mentioned in U.S. Pat. No. 5,764,190, and describes further improvement of the three layered antenna concept. The improvement includes providing of a longitudinal slit in the resonator layer in order to obtain two radiating elements. The RF signal is fed to the radiating elements via the intermediate plate.
A letter by Z. D. Lui and P. S. Hall, "Dual-Frequency Planar Inverted-F Antenna", is published in IEEE Transactions on Antennas and Propagation, October 1997, Volume 45, Number 10. This letter describes a number of solutions--one of these having a rectangular patch for the 900 MHz band. This patch is provided with an L-shaped slot separating one quarter of the 900 MHz band for acting as resonating element in 1800 MHz band. The two resonating elements are interconnected in the bottom of the slot the common feeding point is provided in this interconnection. Furthermore the two resonating elements are shortened in this interconnection by means of a number of shorting pins. Hereby the coupling between the two radiating elements is reduced.
An object of the invention is to provide a dual band antenna for a handset having a form that can be integrated into the per se known type of handset cabinets.
This purpose is obtained by an antenna device having a first metallic layer acting as resonator plane and defining at least two radiating elements separated by a first slit, a second metallic layer arranged substantially in parallel with the first metallic layer and acting as ground plane for the antenna device, feeding means connected to said first and the second metallic layer for feeding an RF signal to the antenna device, said feeding means includes a feeding point in the first metallic layer, and said first metallic layer is provided with a second slit extending from said first slit with the feeding point placed on a metallized tongue between the first and the second slit, whereby surface currents have to pass around the tips of the slits towards the radiating elements. Hereby the antenna device may be provided as a dielectric body having the resonator layer provided as a metallic coating. The dielectric body may then be snapped onto a metallic shield and uses this as ground plane.
The currents are flowing from the metalized tongue toward the two radiating elements, and a cut (defining a neck of the path) is used to separate the current on the two resonating elements. Capacitive couplings are used for tuning the resonance frequencies of the radiating elements.
The one or more capacitive couplings are provided as a metallic coating extending perpendicular to said first and second metallic layers. Actually these couplers are provided as metallic coating on side and end walls of a dielectric body also carrying the radiating elements. In the preferred embodiment the couplers are terminated at a short distance from the ground plane.